July 17, 2014 -- An in-depth investigation has concluded that people who smoke marijuana are much more likely to have paranoia than people who don't use the drug.

The study also identifies psychological factors that can lead to feelings of paranoia in people exposed to the main psychoactive ingredient in marijuana, THC.

The team of researchers, led by Professor Daniel Freeman, PHD, of the University of Oxford, found that worrying, low self-esteem, anxiety, depression, and having a range of unsettling changes in perceptions most likely lead to the feelings of paranoia.

Fearing Harm

A paranoid person is someone who has an unfounded fear that others intend to harm them. Many people have some degree of paranoia. Those who are young, poor, in bad health, contemplating suicide, or using marijuana (also called cannabis) are more prone to have paranoid episodes.

The scientists set out to explore two things:

Firstly, does marijuana cause paranoia?

Secondly, how does it affect the mind in order to cause paranoia?

Injecting THC

They tested 121 participants between the ages of 21 and 50. All of them had taken marijuana at least once before.

None of the participants had a history of mental illness, and all were screened to rule out relevant health conditions. But all of those taking part said they'd felt paranoid at least once in the previous month.

The volunteers were not invited to smoke joints. Instead, the scientists injected some of them with THC in order to ensure the results were as accurate as possible.

Two-thirds of the participants were given THC, and one-third received a placebo.

The amount of THC given was equal to a strong marijuana joint, and the effects lasted about 90 minutes.

Virtual Reality

Immediately after being injected, the volunteers were asked to walk into a hospital cafeteria and buy an item. From there they were taken to a lab, where they wore virtual reality headsets displaying a neutral social situation that didn't have any hostile characteristics.

These experiments were followed up with questionnaires and interviews.

After analyzing the results, the scientists found that THC increased the likelihood of paranoia happening.

Half the participants had paranoid thoughts with THC, compared to just 30% with placebo.

The paranoia declined as the drug left the bloodstream.

The drug also caused a range of other psychological effects: anxiety, worry, lowered mood, negative thoughts about the self, various changes in perception (such as sounds being louder than normal and colors brighter), thoughts echoing, altered perception of time, and poorer short-term memory.

Negative Feelings

The researchers believe the study reinforces the idea that paranoia stems from multiple causes.

They say it's likely that paranoia creeps in because THC increases negative feelings, and the perceptual changes lead to the rise in paranoia. There was no indication that the poorer short-term memory caused the increase in paranoia.

Freeman tells WebMD that young people may be more at risk. "There's certainly evidence that if you use cannabis -- particularly when you're young -- and you use it a lot, that this can put you at risk for later problems."

He says the results don't have any implications for policing, the criminal justice system, or politicians.

"I think what it highlights is that if you have greater confidence in yourself, you improve your self-esteem, and if you try not to worry or ruminate about potential threats in the world... then the effects of the THC should hopefully be less capable of inducing paranoia," he says.

The study was part-funded by the National Institute for Health Research (NIHR) Biomedical Research Centre at South London and Maudsley NHS Foundation Trust and King's College London. It's published in the journal Schizophrenia Bulletin.